The present invention relates generally to video surveillance methods and apparatus and, more particularly, to automatic surveillance systems which detect changes in a field of view over time and indicate an alarm condition accordingly.
The use of video cameras at remote locations for surveillance by video monitors is well known. In some circumstances, constant human supervision or monitoring is required. A typical example of this manual surveillance method would be the remote placement of cameras in a retail store to detect shoplifting; another example would be a camera in a bank activated during working hours to monitor a robbery attempt. Many security systems typically employ a plurality of video cameras situated throughout a facility, with a central monitoring location where a human guard keeps watch. These manual systems are predecessors within the field of the present invention.
Human interaction in surveillance is extremely expensive. In some circumstances, then, it is economical and desirable to replace the human observer with an automatic surveillance system, or at least to alleviate the guard from the burden of constant supervision, freeing him to perform other useful work. Automatic surveillance systems have evolved, therefore, to handle situations which do not require constant human supervision. An example of this application would be the monitoring of an empty room at night, where an automatic system would sense the entry of an intruder and sound an alarm. It may be desired to monitor an outdoor parking lot, or perhaps the entrance or exit of a building. Other applications include monitoring products or workpieces on an assembly line, etc. In a multitude of applications, automatic surveillance methods and systems are more economical and even more reliable than systems requiring constant human interaction.
A common problem encountered by all automatic surveillance systems, both indoor and outdoor, involves false alarms triggered by changes in ambient light intensity. For example, in monitoring an outdoor scene such as a parking lot, a cloud passing overhead may substantially affect ambient light conditions and trigger a false alarm. Even in indoor applications, many offices employ automatic light dimming circuits which dim the lights in the evening, causing problems for automatic surveillance systems.
Attempts to solve the false triggering problem are well documented in the art. One well-known technique involves the use of automatic exposure lenses or cameras to compensate for ambient light intensity variations. Unfortunately, this method is limited to only small variations in intensity. Another alleged solution is proposed by Yoshida in U.S. Pat. No. 4,408,224 (Oct. 4, 1983). Yoshida broadly discloses a surveillance method which includes the comparison of two digitized video image signals taken of a "place scenery" at different points in time. To solve the problem caused by changes in ambient light, Yoshida suggests displacing the capturing of the two video images in time by an amount which is negligible with respect to the ambient changes in brightness. For example, Yoshida suggests that displacing the capture of the images by 15 seconds to 1 minute is suitable to overcome the effects of gradually changing brightness. Unfortunately, this attempt to solve the problem is limited in its usefulness in that it is dependent upon the rate of change of the ambient light intensity. While one time setting may be suitable for slowly changing intensity levels (such as might occur at sunset, dawn, etc.), this same time setting may be unsuitable for rapid changes (such as clouds passing overhead during a thunderstorm, or sudden dimming of lights in an office, etc.).
What is needed, then, is a surveillance system which is not only immune to false alarms caused by changes in ambient light conditions, but also functions independently of the speed with which these ambient changes occur.